Microlithographic projection exposure methods and systems are currently used to fabricate semiconductor components and other finely patterned components. A microlithographic exposure process involves using a mask (reticle) that carries or forms a pattern of a structure to be imaged. The pattern is positioned in a projection exposure system between an illumination system and a projection objective in a region of the object surface of the projection objective. Primary radiation is provided by a primary radiation source and transformed by optical components of the illumination system to produce illumination radiation directed at the pattern of the mask in an illuminated field. The radiation modified by the mask and the pattern passes through the projection objective, which forms an image of the pattern in the image surface of the projection objective, where a substrate to be exposed is arranged. The substrate normally carries a radiation-sensitive layer (photoresist).
When a microlithographic projection exposure system is used in the manufacture of integrated circuits, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the integrated circuit. This pattern can be imaged onto an exposure area on a semiconductor wafer which serves as a substrate. The exposure area is sometimes referred to as a die. A die in the context of integrated circuits is a small block of semiconducting material, on which a given functional circuit is fabricated. A single wafer typically contains a large number of adjacent dies which are successively exposed to an image of the pattern.
In one class of microlithographic projection exposure systems each die is irradiated by exposing the entire pattern of the reticle onto the die at once. Such apparatuses are commonly referred to as wafer steppers.
In alternative exposure systems commonly referred to as step-and-scan apparatus or wafer scanner, each exposure area is irradiated progressively in a scanning operation by moving the mask relative to an illumination beam in an effective object field of the projection objective, and simultaneously moving the substrate relative to the projection beam in the conjugate effective image field of the projection objective in respective scanning directions. The mask is typically held in place by a mask holder, which is movable parallel to the object surface of the projection objective in a scanning apparatus. The substrate is typically held by a substrate holder, which is movable parallel to the image surface in a scanning apparatus. The scanning directions may be parallel to each other or anti-parallel to each other, for example. During the scanning operation, the speed of movement of the mask and the speed of movement of the substrate are interrelated via the magnification ratio β of the projection objective, which is smaller than 1 for reduction projection objectives.
Forming a faithful image of a pattern on the substrate with sufficient contrast typically involves the substrate surface should lying in the focal region of the projection objective during exposure. More specifically, the substrate surface should be arranged in the region of the depth of focus (DOF) of the projection objective, which is proportional to the Rayleigh unit RU defined as RU=λ/NA2, where X is the operating wavelength of the projection exposure system and NA is the image-side numerical aperture of the projection objective. Deep ultraviolet (DUV) lithography λ=193 nm, for example, typically involves a projection objective with a numerical aperture of 0.75 or higher to achieve 0.2 μm or smaller features. In this NA-region, the depth of focus is typically some tenth of a micrometer. In general, the depth of focus tends to decrease as the resolving power of the projection system is increased.
It has long been recognized that systems having relatively narrow depth of focus may involve special technical measures to ensure that the exposure area on the substrate is in focus during exposure.
Imaging errors may also be introduced as a result of gravity falses causing a mask shape to deviate from a planar shape. This effect is frequently referred to as “reticle bending”.